Field of the Invention
The present invention concerns a method for establishing a resonant frequency for the operation of a magnetic resonance apparatus in order to acquire magnetic resonance data, as well as a magnetic resonance apparatus that operates according to such a method.
Description of the Prior Art
In magnetic resonance examinations, a resonant frequency dependent on the nuclear spin from which signals are to be obtained, energy at the Larmor frequency is radiated into the examination object. Although the resonant frequency is approximately known, the SNR falls very rapidly with increasing distance from the resonant frequency. Since the resonant frequency also depends on the examination object, it has to be re-determined for each change of the examination object or during changes in position of the examination object.
If a spin is located in different chemical environments, as a result of what is known as the chemical shift effect, a number of resonant frequencies can exist. With protons there are at least three distinguishable resonances for protons in water, silicon and fat. The spacing of the resonant frequencies is given by:δ=(νSS−νRef)/νRef.
This is specified in ppm and is therefore independent of the field strength B0 of the magnetic field.
For hydrogen spins, i.e. protons, the resonant frequency at 1.5 T is approximately 63.5 MHz. At this field strength the spacing of the resonant frequency of fat protons to that of water protons amounts to 225 Hz or field-strength-independent 3.5 ppm, silicon protons have a spacing of 5.0 ppm and at 1.5 T thus of 320 Hz. Spins able to be distinguished in this way are also called spin species. Accordingly □SS designates the frequency of the respective spin species. A proton signal can thus be obtained in vivo from at least three spin species. In this case distinguishing between in vivo or also in vitro and phantom experiments is important to the extent that basically any given amount of substances are able to be mixed in phantom experiments and thus resonant frequencies are also able to occur.
Before the beginning of an examination, adjustment measurements are carried out for homogenizing the magnetic field in the examination volume, so-called shimming, and for establishing the resonant frequency or resonant frequencies, in order to adapt the transmit frequency of the radio-frequency coils. In such cases a system frequency can be established or specified, from the standpoint of which all other relevant frequencies are seen as fixed. For example, the resonant frequency of water protons can be specified as the system frequency. Then the frequency of the fat or silicon protons, where present, is likewise determined with certain restrictions. But this also means that, if only one resonant peak is present in an adjustment measurement spectrum, the spin species to which the resonant peak belongs must also be determined for a part of experiments, for example experiments with fat suppression. Otherwise, pulses limited in bandwidth are used for frequencies for which no resonant spins are present in the examination object.
It is known that the resonant frequencies can be determined semi-automatically. In such cases a spectrum is acquired and a user is asked whether silicon is present in the examination object. Depending on the user input, model spectra with one or two resonance peaks are selected and a calculation of cross-correlation coefficients of the model spectra with the assumed spectrum is carried out. The cross-correlation coefficient with the highest numerical value gives the best match. In this manner the resonant frequency of the spin species present in the examination object can be established.
Each spin species, as described above, basically has a peak. In such cases the spins designated as fat protons can have a number of peaks, of which one is dominant. When “one” fat peak or resonance signal is referred to below, this formulation does not exclude the presence of further peaks. It means that only one peak is relevant for the inventive method.
A disadvantage of the known method is that a user input is necessary. Not only is this is inconvenient; but also a manual entry is always a source of errors. For example, the user can make a mistake or type the entry incorrectly.